In recent years, in order to decrease the weight of automobiles, efforts are being made at reducing the thickness of steel materials which are used in automobiles by increasing their strength. However, this causes some problems as described below, with taking steel sheet, which is representative of steel materials, as an example. When steel sheet is press formed by drawing, for example, as the strength of the steel sheet which is used increases, the contact pressure of the sheet with the die at the time of drawing increases. As a result, galling of the steel sheet or breakage of the steel sheet may occur. If the blank holding force is decreased in order to increase the amount of the material which enters the die at the time of drawing of a steel sheet, with the intention of even slightly lessening this problem, another problem such as variation in the shape after press forming occurs.
Furthermore, so-called springback occurs at the time of press forming. There are countermeasures against this such as the use of a lubricant. However, with high tensile steel sheet having a strength of 780 MPa or above, the effect of such countermeasures is small.
Thus, the press forming of high tensile steel sheet has currently many problems. Hereinafter, this type of material will be referred to as a “difficult-to-press form steel material”.
As a technique for press forming such difficult-to-press form steel materials, it is conceivable to subject a steel material to press forming after the material has been heated. Such a technique includes so-called hot press forming and warm press forming. These two methods will hereinafter be collectively referred to simply as “hot press forming”.
As disclosed in U.K. Patent 1,490,535, according to hot press forming, it is possible to form a steel sheet into a complicated shape with good dimensional accuracy since the steel sheet is softer and more ductile at a high temperature. Another advantage of hot press forming is that strengthening of the steel sheet due to martensite transformation (so-called hardening) can be simultaneously achieved by heating the steel sheet to the austenite region and then performing quenching at the same time as press forming in the die.
However, since hot press forming is a method in which a heated steel sheet is subjected to working, the surface of the steel sheet to be worked is unavoidably oxidized. Even if the steel sheet is heated in a non-oxidizing atmosphere in a heating furnace, the sheet has a possibility of contacting the atmosphere, for example, when it is removed from the furnace before press forming, resulting in the formation of iron oxides on the surface of the steel sheet. These iron oxides have the problem that they may fall off during press forming and adhere to dies, thereby decreasing productivity, or that an oxide film (scale) made from such iron oxides remains on a product produced by press forming and worsens its appearance. Furthermore, if such an oxide film remains on a press formed product, in the case where the product is subsequently coated with a paint, the resulting painted film has poor adhesion to the steel sheet.
Therefore, after hot press forming, it is necessary to remove a layer of iron oxide, which is a constituent of such an oxide film, by applying shot blasting to the press formed product. This unavoidably results in cost increases. In addition, even if the iron oxide layer is removed, the steel sheet by itself has poor rust preventing properties.
Even if a low alloy steel or stainless steel is used so as to prevent the formation of such an oxide film during heating prior to hot press forming and so as to guarantee corrosion resistance, it is impossible to entirely prevent the formation of an oxide film, and the costs become much higher than for plain steel.
In order to prevent such surface oxidation of steel sheet at the time of hot press forming, theoretically it is effective to use a non-oxidizing atmosphere for both the atmosphere at the time of heating and the atmosphere during the entire pressing process, but this results in a large increase in equipment costs.
Due to these circumstances, even today, hot press forming is not sufficiently utilized.
An outline of current technology which has been proposed in patent applications is as follows.
One advantage of hot press forming is that heat treatment can be performed simultaneously with press forming. It is proposed in JP 07-116900A (1995) to simultaneously perform surface treatment at this time. However, there is no disclosure therein with respect to a means of solving the above-described problems due to surface oxidation.
A steel sheet for hot working is proposed in JP 2000-38640A, which is coated with aluminum in order to provide the steel sheet with resistance to oxidation at the time of hot working. However, this steel sheet, too, is much more expensive than plain steel.
As proposed in JP 06-240414A (1994), from just the standpoints of improving rust preventing properties or corrosion resistance, the addition of elements such as Cr and Mo to the steel composition of a steel material is employed in some cases. However, with such a countermeasure, not only do the costs increase due to the addition of Cr and Mo, but in the case of a material for press forming, there is the problem of a deterioration in press formability due to the addition of these alloying elements.